In this article we will discuss about:- 1. Meaning of Lichens 2. Classification of Lichens 3. Occurrence 4. Internal Structure.
Meaning of Lichens:
Lichens is a combination of two different members belonging to two different groups, i.e., one is a fungal component while the another is an algal component. So, it is a dual organism. Algal component is called phycobiont while the fungal component is known as mycobiont.
About 400 genera and more than 15000 species of lichens have been reported so far, and now it has become a separate branch of Botany with the name Lichenology. Some common lichens are shown in Fig. 117.
Systematic position of lichens have always been a controversial point only due to their dual nature. Bessey (1950) has placed them in order Lecanorales of Ascomycetes while Bold (1959) has proposed a new name Mycophycophyta (mykes-fungi; phykos-algae; phyta-plants) due to the dual nature of the organisms.
Regarding the nature of the lichens, there are various schools of thoughts. Some are of the opinion that fungus lives as parasite on algal partner and so alga is simply a victim of the fungus, while others opine that the two partners of the combination remain in symbiotic relationship helping each other in some or other ways.
A few lichenologists are of the opinion that, of course, there is the symbiotic relationship between two partners but the fungal partner has its upper hand in the partnership, and thus alga lives as prisoner, and this phenomenon has been termed as helotism.
Most of the fungal members in the lichens belong to class Ascomycetes, except a few which belong to Basidiomycetes. Algal members belonging to lichens are mostly the members of class Cyanophyceae, except a few of Chlorophyceae.
Classification of Lichens:
Lichens have been classified on the basis of their fungal members and nature of their fruiting body as follows:
Ascolichens are further divisible into:
(i) Gymnocarpae- (fruiting body apothecium)
(ii) Pyrenocarpae- (fruiting body perithecium)
Occurrence of Lichens:
Lichens occur commonly on tree trunks or bare rocks, old walls or roof of houses in hilly regions, and on the basis of their habitat they have been classified into saxicoles and corticoles. Saxicoles lichens occur on stones or rocks while the corticoles occur on bark of the trees.
Lichens are thallus-like bodies, variously lobed and flat or sometimes cylindrical or erect structures, of various colours.
On the basis of their form, following five categories of thallus organization of lichens are recognized by Hawksworth and Hill (1984):
i. Leprose Lichens:
In this simplest form of thallus organization of lichens, the fungal hyphae envelope either one or only a very small number of algal cells. A distinct fungal layer does not surround the algal cells all over. The so-formed simple lichen thallus develops superficially over the substratum, provides a powdery appearance, and is called leprose lichen, e.g., Lepraria incana.
ii. Crustose Lichens:
These are very closely adhered to the substratum (Fig. 118) on which they are present, and give a crust-like appearance. It is very difficult to separate them from their substratum. Fruiting bodies are present on the upper surface, e.g., Graphis scripta, Lacidia, Verrucaria, etc.
iii. Foliose Lichens:
Thallus in these lichens is flat, leaf-like, well-branched and attached to the substratum with the help of rhizines, e.g., Physcia, Parmelia, Peltidea, etc. (Fig. 119).
iv. Fruticose Lichens:
Lichens of this category are well-branched structures (Figs. 120,121), which are generally erect or sometimes prostrate, and give shrub like appearance, e.g., Usnea, Cladonia, Everinea, etc.
v. Filamentous Lichens:
In some lichens, instead of fungal, the algal partner is more developed. Such algal partners of the lichens are filamentous and remain ensheathed or covered by only a few fungal hyphae. Such lichens are filamentous in appearance, have the dominance of algal partner, and are named filamentous lichens by Hawks-worth and Hill (1984), e.g., Ephebe, Coenogonium, Cystocoleus and Racodium.
Internal Structure of Lichens:
Internally, a foliose lichen is divisible into four different regions, i.e., upper cortex, algal zone, medulla and lower cortex (Fig. 122).
Upper cortex is a thick, protective layer made up of dense, closely woven pseudoparenchymatous mass of fungal hyphae. Outer to this, an epidernal layer may or may not be present.
Algal zone consists of mostly blue-green, or green algal filaments of Cyanophyceae or Chlorophyceae. Some fungal hyphae also remain embedded in this layer. Common algal members are Nostoc, Gloeocapsa, Rivularia and Chlorella. This is also known as gonidial layer.
Medulla forms the central region of the lichen. It consists of loose mass of fungal hyphae.
Lower cortex forms the lower surface of lichen and from this arises rhizines. It is composed of densely packed hyphae.
A crustose lichen (Fig. 123) is internally differentiated into cortex, algal zone and medulla. Some hyphae of medulla form pointed rhizoids.
1. Apothecium is a cup-shaped (Fig. 124) body.
2. In the cavity of the cup are present many asci and sterile paraphyses.
3. In each ascus are present generally eight uninucleate ascospores.
4. Other structures of the internal organization are same as that of vegetative thallus.